Flexion mode crystalline bar for an oscillator

ABSTRACT

Crystalline parallelepiped bar oscillating piezoelectrically in a plane, in a flexion mode, whose terminal faces, are inclined by respect to the plane perpendicular to the longitudinal axis of symmetry of the bar, at an angle ranging between 0* and 60* measured in the flexion plane, and at an angle ranging between 0* and 45* measured in the plane perpendicular to the flexion plane.

United States Patent [191 Gibert et al.

[ Jan. 22, 1974 FLEXION MODE CRYSTALLINE BAR FOR AN OSCILLATOR I [75] Inventors: Guy Gibert; Pierre Vidal, both of Sartrouville, France [73] Assignee: Compagnie dElectronique et de Piezo Electricite C.E.P.E., Sartrouville, France 22 Filed: Feb. 9, 1972 21 Appl. No.: 224,704

[30] Foreign Application Priority Data Feb. 26, 1971 France 7106696 [52] US. Cl 3l0/9.5, 310/82, 310/94,

310/96 [51] Int. Cl H04! 17/00 [58] Field of Search 310/82, 9.5, 9.6, 9.1, 9.4

[56] References Cited UNITED STATES PATENTS Erwin 310/95 X 2/1960 Franx 310/95 3/1967 Mason 310/96 X Primary Examiner-J. D. Miller Assistant Examiner-Mark O. Budd Attorney, Agent, or FirmJ0hn W. Malley et a1.

[ 7] ABSTRACT Crystalline parallelepiped bar oscillating piezoelectrically in a plane, in a flexion mode, whose terminal faces, are inclined by respect to the plane perpendicular to the longitudinal axis of symmetry of the bar, at an angle ranging between 0 and 60 measured in the flexion plane, and at an angle ranging between 0 and 45 measured in the plane perpendicular to the flexion plane.

3 Claims, 3 Drawing Figures FLEXION MODE CRYSTALLINE BAR FOR AN OSCILLATOR electrically sustained by piezoelectric effect through I suitably disposed electrodes, and the material of which they are made being usually constituted by a quartz crystal, cut in particular directions which are associated with its crystal lattice.

In the case of the low frequencies, ranging for example from some few kilohertz per second up to some few tens of megahertz per second, the crystal elements used take the form of elongated bars whose section is nearsquare, equipped with electrodes whose shape and disposition are such that the resultant vibrations involve a bending or flexion of the bar.

However, the need to attach the vibrating crystal in mountings, normally by soldering operations, creates a substantial difficulty, namely that of the transmission of mechanical energy fromthe crystal to its mountings or supporting arrangements, and this in turn means that modifications to the operating characteristics of the crystal on its own, are introduced moreover, said modifications develop over a period of time as a consequence of the aging of the soldered joints referred to.

These phenomena can be reduced by selecting, on the crysta'ls,points of attachment for the suspension elements which are special points where the amplitude of the vibration is at a minimum. In the prior art, these points are selected on the longitudinal axes of the fixing faces, at special distances from the ends which, considering the flexion mode of vibration, are made equal to 0.224 times the length of the bar.

However, this usual choice is based upon calculations for the case of homogeneous oscillatory bars, taking no account of the orientated and anisotropic character of the piezoelectric crystals which are used in actuality, nor in particular of the asymmetric shear effects resulting from the bending deflection. The result is that excessive alternating stresses are developed in the zones comprising the soldered fixings of the bars, with consequent change in their characteristics over a period of time, and ultimately a notable variation of the generated frequency.

The Applicants have proposed in their Patent filed in France under N0. PV 71 05,751 on Feb. 19, 1971 a new device for fixating such a crystal bar in which the positions of each pair of attachment points for the suspension elements are chosen, on the two corresponding suspension faces,at distances from the ends which are respectively greater and less than the indicated value of 0.224 times the length of the bar and, moreover, are outside the longitudinal axes of symmetry of said faces.

Bars equipped with fixings designed in accordance with the invention have exhibited improved characteristics of operation compared with the prior art ones.

However, in the case where the bars have very small dimensions, it has been observed that the residual influence of the fixing devices has a relatively excessive effeet, and a more detailed examination has led to the conclusion that this is attribuable in part to the fact that the, external contour of the bars is established without any reference to the anisotropic nature of their material.

The present invention overcomes these drawbacks, by cutting the crystal bars following new external contours, taking into account the anisotropy of the material of which they are made.

More precisely, the invention relates to an oscillatory bar of parallelepiped form capable of mechanical oscillations in a flexion mode in one plane, said bar comprising two pairs of faces parallel to its longitudinal axis of symmetry, the first pair being perpendicular to said flexion plane and the second parallel thereto, and a pair of terminal faces limited by external straight contour lines, wherein said contour lines contained in the first pair of faces, make an angle ranging between 0 and 60 with a straight line perpendicular to said second pair of faces.

The invention will be better understood from a consideration of the ensuing description and with reference to the attached drawings in which FIG. 1 illustrates a prior art crystal bar FIG. 2 illustrates a first embodiment of a crystal bar in accordance with the invention I FIG. 3 illustrates a second embodiment of a crystal bar in accordance with the invention.

FIG. 1 illustrates a crystal bar, fixed in a partially illustrated support, designed to operate in the flexion mode, said bar being of a conventional kind.

It is cut in the form of a rectangular parallelepiped in a quartz crystal, the corners of said parallelepiped being orientated in special directions associated with thecrystal lattice in order to achieve characteristics 'which accord with the intended mode of oscillation.

Moreover, the crystallographic axes X, Y and Z, being respectively the electric, mechanical and optical axes, it is known to cut the crystal bar so that the directions of its corners are parallel to the axes X, Y and Z the axes Y Y and Z Z making an angle 6 with one another,which is normally close to 5 counted in the trigonometric direction, this kind of cut giving the bar a minimum frequency drift as'a function of temperature. The bar is equipped with fixing devices 4 and 5 the locations of whose points of attachment to the fixing faces, e.g., at 10, are chosen in accordance with French Patent No. PV 71 05,751 dated Feb. 19, 1971 of the present Applicants, namely at the intersection between these faces and a particular straight line D passing through a point M on the longitudinal axis of symmetry of the bar. This point is located at a distance from the nearest end of the bar, which is between 0.2 and 0.224 times the largest dimension of the bar.

The straight line D is on the other hand contained in a plane P perpendicular to the fixing faces and containing the point M the straight lines of intersection with these faces can be considered as lines of minimum vibration,upon which there can be freely selected, in the manner hereinbefore described, the locations of the points of attachment of the device used to fix the crystal.

FIG. 2 illustrates a first embodiment of the oscillating bar in accordance with the invention.

Partly in broken line, as marked by the reference 8, there has been illustrated the parallelepiped contour of a bar of prior art kind, equipped with fixing devices 4 and 5 connected to a support 1, not show in its entirely for the sake of clarity. The terminal faces 6 and 7 of the bar in accordance with the invention differ, however, from those of the known bars, in that they are not perpendicular to the direction of the longitudinal axis of symmetry of the bar, but respectively make a given angle gamma with the perpendicular plane corresponding to the usual terminal faces 9.

This angle, the value ranges between and 45 depends upon the ratio between the thickness 2 and the width w of the bar, and is measured in the reverse direction, namely the clockwise direction. The determination of this angle has been based upon theoretical considerations checked by practical trials, from considerations similar to that which have led to the adoption of new positions for the fixing points, this explaining why in both cases, the senses of the angles alpha relating to the straight lines linking the pairs of points, and the senses of the angles gamma relating to the terminal faces, are the same.

FIG. 3 illustrates a second embodiment of an oscillatory crystal bar in accordance with the invention. It exhibits the same general geometric features as that of FIG. 2, but the terminal faces planes have additionally been rotated through an angle delta about the axis Z.

This angle, whose value ranges between 0 and 60", is measured in the plane parallel to the plane X Y in the reverse direction, namely the clockwise direction.

This rotation angle tends, in the crystal, to make the directions of the terminal faces and those of the lines of minimum vibration of the crystal, already referred to, to approach one another, in magnitude and direction,thus satisfying over the whole external surface, the limit conditions characterising a given mode of vibration in an anisotropic medium.

By way of example, a quartz crystal bar, cut in accordance with the invention, having a length of 47 mm, a thickness of 2.4 mm and a width of 1.6 mm, with termina] faces having gamma and delta angles of 1 l and 40 respectively had an aging rate four times less than that exhibited by a crystal bar cut in accordance with the prior art.

Of course, the invention is not limited to the embodiment described and shown which was given solely by way of example.

What is claimed is l. A crystalline bar of parallelepiped form, capable of piezoelectric oscillations in a plane, in a flexion mode, comprising two pairs of faces parallel to its longitudinal axis of symmetry, the first pair being perpendicular to said plane, and the second parallel thereto, and one pair of terminal faces limited by external straight contour lines, said second pair including the fixing points for attaching at zones of minimum flexion vibration one of the ends of bar suspension wires respectively attached by their other end to a support, each of said pairs being equipped with a pair of plane electrodes, each pair of electrodes being respectively connected to one of the two terminals of an alternative voltage source, wherein those of said straight contour lines contained in the first pair of faces make an angle of between 0 and 45 with a direction perpendicular to said second pair of faces, counted in a clockwise direction.

2. A crystalline bar as claimed in claim 1, wherein those of said straight contour lines contained in the second pair of faces make an angle ranging between 0 and 60, with a direction perpendicular to said first pair of faces counted in a clockwise direction.

3. A crystal oscillator comprising a crystalline bar as claimed in claim 1. 

1. A crystalline bar of parallelepiped form, capable of piezoelectric oscillations in a plane, in a flexion mode, comprising two pairs of faces parallel to its longitudinal axis of symmetry, the first pair being perpendicular to said plane, and the second parallel thereto, and one pair of terminal faces limited by external straight contour lines, said second pair including the fixing points for attaching at zones of minimum flexion vibration one of the ends of bar suspension wires respectively attached by their other end to a support, each of said pairs being equipped with a pair of plane electrodes, each pair of electrodes being respectively connected to one of the two terminals of an alternative voltage source, wherein those of said straight contour lines contained in the first pair of faces make an angle of between 0* and 45* with a direction perpendicular to said second pair of faces, counted in a clockwise direction.
 2. A crystalline bar as claimed in claim 1, wherein those of said straight contour lines contained in the second pair of faces make an angle ranging between 0* and 60*, with a direction perpendicular to said first pair of faces counted in a clockwise direction.
 3. A crystal oscillator comprising a crystalline bar as claimed in claim
 1. 